Thin-film transistor, method of manufacturing the same, and electronic device

ABSTRACT

A thin-film transistor includes: an organic semiconductor layer; and a source electrode and a drain electrode spaced apart from each other and disposed to respectively overlap the organic semiconductor layer. The organic semiconductor layer INCLUDES: a lower organic semiconductor layer; and an upper organic semiconductor layer formed on the lower organic semiconductor layer and having solubility and conductivity higher than the lower organic semiconductor layer. The lower organic semiconductor layer extends from an area overlapping the source electrode to an area overlapping the drain electrode, while the upper organic semiconductor layer is disposed in each of the area overlapping the source electrode and the area overlapping the drain electrode so that the respective upper organic semiconductor layers are spaced apart from each other.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Japanese Priority PatentApplication JP 2010-022161 filed in the Japan Patent Office on Feb. 3,2010, the entire content of which is hereby incorporated by reference.

BACKGROUND

The present application relates to a thin-film transistor having anorganic semiconductor layer, a method of manufacturing the thin-filmtransistor, and an electronic device using the thin-film transistor.

In recent years, an active matrix drive system has been introduced intomany electronic devices represented by a display device, and a thin-filmtransistor (TFT) is used as an element for the switching (pixelselection).

FIG. 9 illustrates a cross-sectional configuration of a channel etchtype of TFT in the past. In this TFT, a semiconductor layer 103 isformed on a gate electrode 101 and a gate insulating layer 102, and asource electrode 104 and a drain electrode 105 are connected to thesemiconductor layer 103. The source electrode 104 and the drainelectrode 105 are spaced apart from each other and respectively disposedto be overlaid on an upper part of the semiconductor layer 103.

Regions R1 to R3 illustrated in FIG. 9 represent regions determined by apositional relationship between the semiconductor layer 103 and thesource electrode 104 as well as the drain electrode 105. The region R1is where the semiconductor layer 103 overlaps the source electrode 104,and the region R2 is where the semiconductor layer 103 overlaps thedrain electrode 105. Further, the region R3 is located between theregions R1 and R2, and where the semiconductor layer 103 overlapsneither the source electrode 104 nor the drain electrode 105. What ismeant by these regions R1 to R3 remains the same hereinafter.

The semiconductor layer 103 which is a channel layer has a layeredstructure in which an upper semiconductor layer 103B is formed on alower semiconductor layer 103A. The lower semiconductor layer 103A isformed of amorphous silicon, and extends from the region R1 via theregion R3 to the region R2. The upper semiconductor layer 103B is formedof amorphous silicon doped n-type and is disposed on each of the regionsR1 and R2 so that the respective upper semiconductor layers 103B arespaced apart from each other.

When such a semiconductor layer 103 is formed, first, the lowersemiconductor layer 103A and the upper semiconductor layer 103B areformed to extend from the region R1 to the region R2 and then, thesource electrode 104 and the drain electrode 105 are formed.Subsequently, the upper semiconductor layer 103B is selectively etchedby using the source electrode 104 and the drain electrode 105 as a mask.As a result, a part of the upper semiconductor layer 103B is removed inthe region R3 and thus, the upper semiconductor layer 103B remains onlyin each of the regions R1 and R2.

In this channel etch type of TFT, electrical resistance of then-type-doped upper semiconductor layer 103B becomes lower thanelectrical resistance of the lower semiconductor layer 103A not dopedn-type. As a result, contact resistance between the source electrode 104as well as the drain electrode 105 and the upper semiconductor layer103B decreases and thus, an electric charge becomes easy to go in andout between the source electrode 104 as well as the drain electrode 105and the semiconductor layer 103.

Incidentally, an organic TFT using the organic semiconductor layer as achannel layer has been receiving attention recently. In the organic TFT,the channel layer can be formed by coating, which makes it possible toreduce the cost. In addition, the channel layer can be formed at atemperature lower than an evaporation method and the like and thus, theorganic TFT can be implemented on a low heat-resistant flexible plasticfilm or the like.

FIG. 10 illustrates a cross-sectional configuration of the organic TFTin the past. This organic TFT has a structure similar to the TFTillustrated in FIG. 9, except that has an organic semiconductor layer203 in replace of the semiconductor layer 103. Specifically, the organicsemiconductor layer 203 is formed on a gate electrode 201 and a gateinsulating layer 202, and a source electrode 204 and a drain electrode205 are connected to the organic semiconductor layer 203. This organicsemiconductor layer 203 has a single-layered structure, and extends fromthe region R1 to the region R2.

As a structure of this organic TFT, like TFTs in the past, various kindsof structure such as a top contact type, a bottom contact type, a topgate type and a bottom gate type are considered. Above all, the topcontact type in which the source electrode and the drain electrode aredisposed to be overlaid on an upper part of the organic semiconductorlayer is common (for example, see International PublicationWO2007/055119)

SUMMARY

In order to make use of an advantage of the organic TFT to thereby makean electronic device thinner and more flexible, it is strongly desiredto improve the performance of the organic TFT, in particular, themobility and the ON/OFF ratio.

Thus, application of the structure of the channel etch type illustratedin FIG. 9 to the organic TFT has been studied, but this attempt is in adifficult situation resulting from the channel layer being an organicsemiconductor. This is because, in order to remove the organicsemiconductor, oxygen plasma etching to burn an organic matter isgenerally used, but in this etching, it is difficult to etch the organicsemiconductor (only a necessary part) selectively. In this case, if theorganic semiconductor composed of two layers is formed and then only anupper layer is etched, a lower layer is not a little etched. As aresult, the thickness of the lower layer decreases and at the same time,the property deteriorates due to damage caused at the time of etching.

In view of the foregoing, it is desirable to provide a thin-filmtransistor, a method of manufacturing the thin-film transistor, and anelectronic device, which can improve performance of the thin-filmtransistor.

According to an embodiment, there is provided a thin-film transistorincluding: an organic semiconductor layer; and a source electrode and adrain electrode spaced apart from each other and disposed torespectively overlap the organic semiconductor layer. This organicsemiconductor layer includes a lower organic semiconductor layer, and anupper organic semiconductor layer formed on the lower organicsemiconductor layer and having solubility and conductivity higher thanthe lower organic semiconductor layer. The lower organic semiconductorlayer extends from a region overlapping the source electrode to a regionoverlapping the drain electrode, while the upper organic semiconductorlayer is disposed in each of the region overlapping the source electrodeand the region overlapping the drain electrode so that the respectiveupper organic semiconductor layers are spaced apart from each other.Further, an electronic device according to an embodiment includes theabove-described thin-film transistor.

According to an embodiment, there is provided a method of manufacturinga thin-film transistor and including steps of: forming a lower organicsemiconductor layer; and forming an upper organic semiconductor layer onthe lower organic semiconductor layer, the upper organic semiconductorlayer having solubility and conductivity higher than those of the lowerorganic semiconductor layer. Further, the method includes steps of:forming a source electrode and a drain electrode spaced apart from eachother and respectively overlapping the upper organic semiconductorlayer; and dissolving the upper organic semiconductor layer selectivelyby using the source electrode and the drain electrode as a mask.

According to the thin-film transistor and the method of manufacturingthe same, as well as the electronic device of the embodiments, the upperorganic semiconductor layer has the solubility and the conductivityhigher than those of the lower organic semiconductor layer. In thiscase, the upper organic semiconductor layer is removed selectively bysimple dissolution processing using a solvent such as an organicsolvent, even if oxygen plasma etching is not used. Thus, the lowerorganic semiconductor layer is formed to extend from the regionoverlapping the source electrode to the region overlapping the drainelectrode, and the upper organic semiconductor layer is formed in eachof the region overlapping the source electrode and the regionoverlapping with the drain electrode so that the respective upperorganic semiconductor layers are spaced apart from each other.Therefore, a channel etch type of thin-film transistor provided with theorganic semiconductor layer is produced easily with stability.Accordingly, the mobility and the ON-OFF ratio improve and thus, theperformance can be improved.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a cross-sectional diagram illustrating a configuration of athin-film transistor in one embodiment;

FIG. 2 is a cross-sectional diagram intended for description of a methodof manufacturing the thin-film transistor;

FIG. 3 is a cross-sectional diagram intended for description of aprocess subsequent to FIG. 2;

FIG. 4 is a cross-sectional diagram illustrating a configuration of amain part of a liquid crystal display device that is an applicationexample of the thin-film transistor;

FIG. 5 is a diagram illustrating a circuit configuration of the liquidcrystal display device illustrated in FIG. 4;

FIG. 6 is a cross-sectional diagram illustrating a configuration of amain part of an organic electroluminescent (EL) display device that isan application example of the thin-film transistor;

FIG. 7 is a diagram illustrating a circuit configuration of the organicEL display device illustrated in FIG. 6;

FIG. 8 is a cross-sectional diagram illustrating a configuration of amain part of an electronic paper display device that is an applicationexample of the thin-film transistor;

FIG. 9 is a cross-sectional diagram illustrating a configuration of athin-film transistor in the past; and

FIG. 10 is a cross-sectional diagram illustrating a configuration of anorganic thin-film transistor in the past.

DETAILED DESCRIPTION

Embodiments of the present application will be described below in detailwith reference to the drawings.

1. Thin-film transistor (organic TFT) and method of manufacturing thesame

2. Application example (electronic device) of thin-film transistor(organic TFT)

2-1. Liquid crystal display device

2-2. Organic EL display device

2-3. Electronic paper display device

1. Organic TFT and Method of Manufacturing the Same

Entire Configuration of Thin-Film Transistor

FIG. 1 illustrates a cross-sectional configuration of an organic TFTthat is a thin-film transistor in one embodiment.

The organic TFT is a TFT in which an organic semiconductor layer 3 isdisposed to face a gate electrode 1 via a gate insulating layer 2, and asource electrode 4 and a drain electrode 5 are connected to the organicsemiconductor layer 3. The source electrode 4 and the drain electrode 5are spaced apart (separated) from each other and disposed torespectively overlap the organic semiconductor layer 3.

The organic TFT described here is a top-contact bottom-gate type inwhich the gate electrode 1 is located lower than the organicsemiconductor layer 3, and the source electrode 4 and the drainelectrode 5 are overlaid on the organic semiconductor layer 3.

The gate electrode 1 is formed of, for example, one or more kind ofmetallic material, inorganic conductive material, organic conductivematerial, and carbon material. The metallic material is, for example,aluminum (Al), copper (Cu), molybdenum (Mo), titanium (Ti), chromium(Cr), nickel (Ni), palladium (Pd), gold (Au), silver (Ag), platinum(Pt), or an alloy including any of them. The inorganic conductivematerial is, for example, indium oxide (In₂O₃), indium tin oxide (ITO),indium oxide zinc (IZO), or zinc oxide (ZnO). The organic conductivematerial is, for example, polyethylene dioxythiophene (PEDOT), orpolystyrene sulfonate (PSS). The carbon material is graphite or thelike. Incidentally, the gate electrode 1 may be formed of two or morelaminated layers made of any of the above-described various kinds ofmaterial, and the gate insulating layer 2, the organic semiconductorlayer 3, the source electrode 4 and the drain electrode 5 to bedescribed later also may be similarly formed of such lamination.

The gate insulating layer 2 is formed of, for example, one or more kindof inorganic insulating material or organic insulating material. Theinorganic material is, for example, silicon oxide (SiOx), siliconnitride (SiNx), aluminum oxide (Al₂O₃), titanium oxide (TiO₂), hafniumoxide (HfOx), or barium titanate (BaTiO₃). The organic insulatingmaterial is polyvinyl phenol (PVP), polyimide, polymethacrylic acidacrylate, photosensitive polyimide, photosensitive novolac resin,poly-para-xylylene, or the like.

The organic semiconductor layer 3 has a layered structure in which anupper organic semiconductor layer 3B is formed on a lower organicsemiconductor layer 3A. Incidentally, each of the lower organicsemiconductor layer 3A and the upper organic semiconductor layer 3B maybe a single layer or a multilayer.

The lower organic semiconductor layer 3A extends from a region R1overlapping the source electrode 4 to a region R2 overlapping the drainelectrode 5, and is formed of one or more kind of organic semiconductormaterial. Such an organic semiconductor material is acene, itsderivative, or the like. The acene is naphthacene, pentacene, hexacene,heptacene, dibenzo pentacene, tetrabenzo pentacene, pyrene,dibenzopyrene, chrysene, perylene, coronene, Terylene, ovalene,quaterrylene, circumanthracene, or the like. The derivative of the aceneis, for example, a material in which a part of carbon (C) is replacedwith other element such as nitrogen (N), sulfur (S), or oxygen (O), or amaterial in which the group is partially replaced with a functionalgroup such as a carbonyl group. Specific examples of this derivativeinclude triphenodioxazine, triphenodithiazine, andhexacene-6,15-quinone. In addition, the organic semiconductor materialis, for example, anthradithiophene,dinaphto[2,3-b:2′,3′-f]thieno[3,2-b]thienophen, peri-xanthenoxanthenecompound such as 2,9-diphenyl-peri-xanthenoxanthene, or2,9-dinaphthyl-peri-xanthenoxanthene, or copper phthalocyanine or thelike.

The upper organic semiconductor layer 3B is disposed in each of the areaR1 overlapping with the source electrode 4 and the area R2 overlappingwith the drain electrode 5, so that the respective upper organicsemiconductor layers 3B are spaced apart from each other. The upperorganic semiconductor layer 3B has solubility and conductivity higherthan those of the lower organic semiconductor layer 3A.

The “solubility” mentioned above is the solubility in a solvent such asan organic solvent. In addition, the “high solubility” is the solubilityto obtain, when the lower organic semiconductor layer 3A and the upperorganic semiconductor layer 3B are caused to contact a solvent, such aselectivity that the upper organic semiconductor layer 3B is dissolved(removed) whereas the lower organic semiconductor layer 3A is notdissolved (remain as it is). The materials of the lower organicsemiconductor layer 3A and the upper organic semiconductor layer 3B (thecombination of the organic semiconductor materials) are selected so thatthe above-described selectivity for the solubility is obtained.

This upper organic semiconductor layer 3B is formed of, for example, oneor more kind of organic semiconductor material doped to achieve lowresistance. The organic semiconductor material (main material) beforethe doping is, for example, α-quaterthiophene, poly-(β-hexylthiophene),orpoly(2,5-bis(3-dodecyl-5-(3-dodecylthiophene-2-yl)thiophene-2-yl)thiazolo[5,4-d]thiazole.A doping material is, for example, a charge-transfer complex such astetracyano quinodimethan (TCNQ), or tetrafluoro TCNQ (F4-TCNQ), which isa p-type doping material. The ionization potential of the upper organicsemiconductor layer 3B is desired to be smaller than the ionizationpotential of the lower organic semiconductor layer 3A. This is becausean electric charge is made to easily move inside of the organicsemiconductor layer 3.

Above all, it is desirable that the material (main material) of formingthe upper organic semiconductor layer 3B be a material into which asubstituent to give the solubility to the material of forming the lowerorganic semiconductor layer 3A (a derivative of the material of formingthe lower organic semiconductor layer 3A) is introduced. This is becausewhen the solubility of a material having no substituent is sufficientlylow, the difference in solubility to a material having a substituent islarge and thus, a sufficient selectivity is achieved.

For instance, the material of forming the lower organic semiconductorlayer 3A is pentacene expressed in chemical formula (1), the material(main material) of forming the upper organic semiconductor layer 3B is aderivative of the pentacene expressed in chemical formula (2). “i-Pr”shown in chemical formula (2) represents an isopropyl group. Thisderivative is a material in which two triisopropylsilylethynyl (TIPS)groups are introduced into the pentacene as a side chain. Incidentally,the introduction position and the number of TIPS groups are not limitedto the case shown in chemical formula (2), and can be modified freely.

Here, as the solvent mentioned above, there is dichloromethane,Tetralin, xylene, or the like. In this case, the solubility of pentacenein the solvent is approximately zero, and the pentacene hardlydissolves, but the solubility of the derivative in the solvent becomesdrastically higher than the pentacene, and the derivative dissolveseasily. The combination of these pentacene and derivative is desirablealso in the sense that while the derivative is soluble, both can beformed by using vacuum deposition or the like.

The source electrode 4 and the drain electrode 5 are formed of, forexample, the same material as the gate electrode 1 mentioned above, andis desired to be in ohmic contact with the organic semiconductor layer3.

Incidentally, the organic TFT may include a component other than thosedescribed above. As such other component, there is, for example, asubstrate that supports the organic TFT. This substrate may be, forexample, a substrate made of glass, a plastic material, a metallicmaterial, or the like, or may be a film made of the plastic material,the metallic material or the like, or may be a sheet of paper (generalpaper). The plastic material is polyethersulfone (PES), polycarbonate(PC), polyimide (PI), polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), polyethyl ether ketones (PEEK), or the like. Themetallic material is aluminum, nickel, stainless steel, or the like.Incidentally, the surface of the substrate may be provided with, forexample, various kinds of layer such as a buffer layer to secureadhesion, and a gas barrier layer to prevent gas emission.

Method of Producing Organic TFT

This organic TFT is produced in the following procedure. FIG. 2 and FIG.3 are intended for description of a method of manufacturing the organicTFT, and illustrate cross-sectional configurations corresponding toFIG. 1. Since the materials of forming a series of components have beenalready described above, their description will be omitted in thefollowing. Incidentally, the method of manufacturing the organic TFT tobe described here is merely presented as an example, and the materialsof forming the respective components and the formation methods can bemodified as appropriate.

First, as illustrated in FIG. 2, the gate electrode 1 is formed. In thiscase, for example, after a metallic material layer is formed, a mask(not illustrated) such as a resist pattern is formed on the metallicmaterial layer. Subsequently, after the metallic material layer isetched by using the mask, the mask after being used is removed byashing, etching or the like. The method of forming the metallic materiallayer is vacuum film formation, coating, plating, or the like. Thevacuum film formation is vacuum deposition, flash evaporation,sputtering, Physical Vapor Deposition (PVD), Chemical Vapor Deposition(CVD), Pulsed Laser Deposition (PLD), arc discharge method, or the like.The coating is spin coating, slit coating, bar coating, spray coating,or the like. The plating is electrolytic plating, electroless plating,or the like. When the resist pattern is formed, for example, after aphotoresist film is formed by application of a photoresist, thephotoresist film is patterned by using photolithography, laser drawing,electron beam lithography, X-rays drawing, or the like. However, theresist pattern may be formed by using resist transfer or the like. Themethod of etching the metallic material layer is, for example, dryetching or wet etching using an etchant solution, and the dry etchingis, for example, ion milling or Reactive Ion Etching (ME). The etchingfor removing the mask is similar. Incidentally, the method of formingthe gate electrode 1 may be, for example, printing such as inkjetprinting, screen printing, gravure printing, or gravure offset printing.In addition, instead of the resist pattern, a metal pattern may beformed as the mask by using a laser ablation method, mask vacuumdeposition, laser transfer, or the like. Of course, in order to form thegate electrode 1, an inorganic conductive material layer, an organicconductive material layer, a carbon material layer, or the like may beformed in place of the metallic material layer.

Subsequently, the gate insulating layer 2 is formed to cover the gateelectrode 1. Formation procedures of this gate insulating layer 2 differaccording to the formation material, for example. The formationprocedure when an inorganic insulating material is used is similar tothe case in which the gate electrode 1 is formed, except that thecoating may be a sol-gel method or the like. The formation procedurewhen an organic insulating material is used is similar to the case inwhich the gate electrode 1 is formed, except that a photosensitivematerial may be patterned by using the photolithography.

Subsequently, on the gate insulating layer 2, the lower organicsemiconductor layer 3A is patterned by using the pentacene illustratedin chemical formula (1). In this case, the procedure similar to the casein which the gate electrode 1 is formed is used. Specifically, after thelower organic semiconductor layer 3A is formed to cover the gateinsulating layer 2, the lower organic semiconductor layer 3A is etchedby using the photolithography.

Subsequently, on the lower organic semiconductor layer 3A, the upperorganic semiconductor layer 3B is patterned by using the derivative ofthe pentacene shown in chemical formula (2) or the like and a dopingmaterial. In this case, the procedure and formation method similar tothose in the case in which the lower organic semiconductor layer 3A isformed, and the upper organic semiconductor layer 3B is formed to extendfrom the region R1 to the region R2.

Incidentally, instead of patterning the lower organic semiconductorlayer 3A and the upper organic semiconductor layer 3B individually, bothmay be patterned at a time. In this case, after the lower organicsemiconductor layer 3A and the upper organic semiconductor layer 3B arelaminated to cover the surface of the gate insulating layer 2, the lowerorganic semiconductor layer 3A and the upper organic semiconductor layer3B may be patterned at a time by using the photolithography, the laserablation method, or the like.

When the lower organic semiconductor layer 3A and the upper organicsemiconductor layer 3B are formed, it is desirable to use, for example,vacuum film formation such as vacuum deposition or sputtering for bothlayers. This is because the upper organic semiconductor layer 3B isdisposed on the lower organic semiconductor layer 3A in a vacuumenvironment and thus, mixing of a foreign matter into an interfacebetween these two layers is less likely to occur and resistance todamage is attained. In this case, the main material and the dopingmaterial may be evaporated together.

Subsequently, an electrode layer 6 is formed to cover the lower organicsemiconductor layer 3A, the upper organic semiconductor layer 3B and theperipheral gate insulating layer 2. This electrode layer 6 is apreparation layer to form the source electrode 4 and the drain electrode5, and as its formation material, the same material as those of thesource electrode 4 and the drain electrode 5 is used. The method offorming the electrode layer 6 is, for example, similar to the case inwhich the gate electrode 1 is formed, and a method less likely to damagethe lower organic semiconductor layer 3A and the upper organicsemiconductor layer 3B is particularly desirable.

Subsequently, the electrode layer 6 is selectively etched, and asillustrated in FIG. 3, the source electrode 4 and the drain electrode 5are formed. In this case, a procedure similar to the case in which thegate electrode 1 is formed is used. Specifically, after a resist patternis formed on the electrode layer 6, the electrode layer 6 is etched byusing the resist pattern as a mask. In particular, as a method ofetching the electrode layer 6, for example, the wet etching or the likewhich is less likely to damage the lower organic semiconductor layer 3Aand the upper organic semiconductor layer 3B is desirable. In this case,the solubility of the upper organic semiconductor layer 3B in an etchantsolution is made to be small to the extent that the solubility can beignored.

Finally, the upper organic semiconductor layer 3B is selectively causedto dissolve with a solvent, by using the source electrode 4 and thedrain electrode 5 as a mask. The kind of this solvent is freelyselectable, according to the formation materials of the lower organicsemiconductor layer 3A and the upper organic semiconductor layer 3B (aselectivity related to the degrees of solubility of both). As a result,as illustrated in FIG. 1, a part of the upper organic semiconductorlayer 3B (a part located in the region R3) is removed, and the upperorganic semiconductor layer 3B remains in only the regions R1 and R2. Inthis case, although the lower organic semiconductor layer 3A is exposedon a region where the upper organic semiconductor layer 3B hasdissolved, the lower organic semiconductor layer 3A hardly dissolvesaccording to the selectivity. This completes the organic TFT.

Operation and Effect Related to Organic TFT and Method of Manufacturingthe Same

According to the organic TFT and the method of manufacturing the samedescribed above, the upper organic semiconductor layer 3B has thesolubility and the conductivity higher than those of the lower organicsemiconductor layer 3A. In this case, the upper organic semiconductorlayer 3B is removed selectively by simple dissolution processing thatemploys a solvent such as an organic solvent, even if the oxygen plasmaetching is not used. Besides, by setting the degrees of solubility ofthe lower organic semiconductor layer 3A and the upper organicsemiconductor layer 3B so that the sufficient selectivity is obtained,only the upper organic semiconductor layer 3B dissolves, while the lowerorganic semiconductor layer 3A hardly dissolves. As a result, the lowerorganic semiconductor layer 3A is formed to extend from the region R1 tothe region R2, and the upper organic semiconductor layer 3B is formed sothat the upper organic semiconductor layers 3B on the regions R1 and R2are spaced apart from each other and therefore, the channel etch type oforganic TFT with the organic semiconductor layer 3 is readily producedwith stability. Accordingly, the mobility and the ON-OFF ratio improveand thus, the performance can be enhanced.

2. Application Example (Electronic Device) of Organic TFT

Next, an application example of the above-described organic TFT will bedescribed. For instance, this organic TFT is applicable to someelectronic devices, as will be described below sequentially.

2-1. Liquid Crystal Display Device

The organic TFT is applied to, for example, a liquid crystal displaydevice. FIG. 4 and FIG. 5 illustrate a cross-sectional configuration anda circuit configuration, respectively, of a main part of the liquidcrystal display device. Incidentally, the device configuration (FIG. 4)and the circuit configuration (FIG. 5) to be described below are merelypresented as examples, and can be modified as appropriate.

The liquid crystal display device described here is, for example, atransmissive liquid crystal display in an active matrix drive systemusing the organic TFT, and the organic TFT is used as a switching (pixelselection) element. In this liquid crystal display device, asillustrated in FIG. 4, a liquid crystal layer 41 is enclosed between adrive board 20 and an opposite board 30.

The drive board 20 is, for example, a board in which an organic TFT 22,a planarizing dielectric layer 23 and a pixel electrode 24 are formed inthis order on one surface of a supporting board 21, and the pluralorganic TFTs 22 and pixel electrodes 24 are arranged in a matrix.However, the number of organic TFTs 22 included in one pixel may be one,or more than one. FIG. 4 and FIG. 5 illustrate, as an example, a case inwhich the one organic TFT 22 is included in one pixel.

The supporting board 21 is formed of, for example, a transmissivematerial such as glass or a plastic material, and the organic TFT 22 hasa configuration similar to the organic TFT described earlier. The typeof the plastic material is, for example, similar to the case describedabove for the organic TFT, and this also applies to a case that will bedescribed below as occasion arises. The planarizing dielectric layer 23is formed of, for example, an insulating resin material such aspolyimide, and the pixel electrode 24 is formed of, for example, atransmissive conductive material such as ITO. Incidentally, the pixelelectrode 24 is connected to the organic TFT 22 through a contact hole(not illustrated) provided in the planarizing dielectric layer 23.

The opposite board 30 is, for example, a board in which an oppositeelectrode 32 is formed all over one surface of a supporting board 31.The supporting board 31 is formed of, for example, a transmissivematerial such as glass or a plastic material, and the opposite electrode32 is formed of, for example, a conductive material such as ITO.

The drive board 20 and the opposite board 30 are disposed to have thepixel electrode 24 and the opposite electrode 32 facing each other withthe liquid crystal layer 41 interposed in between, and bonded togetherwith a sealant 40. The type of liquid crystal molecules included in theliquid crystal layer 41 is freely selectable.

In addition, for example, the liquid crystal display device may includeother components such as a phase difference plate, a polarizing plate,an oriented film and a backlight unit (none of them illustrated).

The circuit for driving the liquid crystal display device includes, forexample, as illustrated in FIG. 5, the organic TFT 22 and a liquidcrystal display element 44 (an element section including the pixelelectrode 24, the opposite electrode 32 and the liquid crystal layer41), and a capacitor 45. In this circuit, plural signal lines 42 arearranged in a row direction, and plural scanning lines 43 are arrangedin a column direction, and at each intersection of them, the organic TFT22, the liquid crystal display element 44 and the capacitor 45 aredisposed. The destinations to which the source electrode, the gateelectrode and the drain electrode in the organic TFT 22 are connectedare not limited to the case illustrated in FIG. 5, and can be setfreely. The signal lines 42 and the scanning lines 43 are connected to asignal line driving circuit (a data driver) and a scanning line drivingcircuit (a scanning driver) which are not illustrated, respectively.

In this liquid crystal display device, when the liquid crystal displayelement 44 is selected by the organic TFT 22, and an electric field isapplied between its pixel electrode 24 and opposite electrode 32, theoriented state of the liquid crystal molecules in the liquid crystallayer 41 changes according to its electric field strength. As a result,the amount of optical transmission (transmissivity) is controlledaccording to the oriented state of the liquid crystal molecules andthus, a tone image is displayed.

According to this liquid crystal display device, the organic TFT 22 hasthe configuration similar to that of the organic TFT described earlierand thus, the mobility and the ON-OFF ratio improve. Therefore, thedisplay performance can be enhanced. Incidentally, the liquid crystaldisplay device is not limited to the transmission type, and may be areflection type.

2-2. Organic EL Display Device

The organic TFT is applied to, for example, an organic EL displaydevice. FIG. 6 and FIG. 7 illustrate a cross-sectional configuration anda circuit configuration, respectively, of a main part of the organic ELdisplay device. Incidentally, the device configuration (FIG. 6) and thecircuit configuration (FIG. 7) to be described below are merelypresented as examples, and can be modified as appropriate.

The organic EL display device described here is, for example, an organicEL display in an active matrix drive system which uses the organic TFTas a switching element. This organic EL display device is configuredsuch that a drive board 50 and an opposite board 60 are bonded togetherwith an adhesive layer 70 and is, for example, a top emission type thatemits light via the opposite board 60.

In the drive board 50, for example, an organic TFT 52, a protectivelayer 53, a planarizing dielectric layer 54, a pixel separationinsulating layer 55, a pixel electrode 56, an organic layer 57, anopposite electrode 58 and a protective layer 59 are formed in this orderon one surface of a supporting board 51. The plural organic TFTs 52,pixel electrodes 56 and organic layers 57 are arranged in a matrix.However, the number of organic TFTs 52 included in one pixel may be one,or more than one. FIG. 6 and FIG. 7 illustrate, as an example, a case inwhich the two organic TFTs 52 (a selection TFT 52A and a drive TFT 52B)are included in one pixel.

The supporting board 51 is formed of glass, a plastic material or thelike. In the top emission type, the light is taken out from the oppositeboard 60 and thus, the supporting board 51 may be formed of either atransmissive material or a non-transmissive material. The organic TFT 52has a configuration similar to that of the organic TFT describedearlier, and the protective layer 53 is formed of, for example, apolymeric material such as poly-vinyl alcohol (PVA) orpoly-para-xylylene. The planarizing dielectric layer 54 and the pixelseparation insulating layer 55 are formed of, for example, an insulatingresin material such as polyimide. For example, it is desirable that inorder to simplify a formation process and to enable formation in adesired shape, this pixel separation insulating layer 55 be formed of aphotosensitive resin material capable of being molded by lightpatterning, reflow or the like. Incidentally, the planarizing dielectriclayer 54 may be omitted when sufficient flatness is achieved by theprotective layer 53.

The pixel electrode 56 is formed of, for example, a reflective materialsuch as aluminum, silver, titanium or chromium, and the oppositeelectrode 58 is formed of, for example, a transmissive conductivematerial such as ITO or IZO. However, the opposite electrode 58 may beformed of, for example, a transmissive metallic material such as calcium(Ca), its alloy, or a transmissive organic conductive material such asPEDOT. The organic layer 57 includes a light-emitting layer that emitslight of red, green or blue, and may have a layered structure includinga hole transport layer, an electron transport layer and the like asneeded. The material of forming the light-emitting layer can be freelyselected according to the color of the light to be emitted. The pixelelectrodes 56 and the organic layers 57 are arranged in a matrix whilebeing spaced with the pixel separation insulating layers 55, whereas theopposite electrode 58 extends continuously while facing the pixelelectrodes 56 via the organic layer 57. The protective layer 59 isformed of, for example, a light-transmissive dielectric material such assilicon oxide (SiOx), aluminum oxide (AlOx), a silicon nitride (SiN),poly-para-xylylene or urethane. Incidentally, the pixel electrode 56 isconnected to the organic TFT 52 through a contact hole (not illustrated)provided in the protective layer 53 and the planarizing dielectric layer54.

The opposite board 60 is, for example, a board in which a color filter62 is provided on one surface of a supporting board 61. The supportingboard 61 is formed of, for example, a transmissive material such asglass, a plastic material or the like, and the color filter 62 hasplural color regions corresponding to the colors of light produced inthe organic layers 57. However, the color filter 62 may be omitted.

The adhesive layer 70 is, for example, an adhesive such as thermosettingresin.

The circuit for driving the organic EL display device includes, forexample, as illustrated in FIG. 7, the organic TFTs 52 (the selectionTFT 52A and the drive TFT 52B) and an organic EL display element 73 (anelement section including the pixel electrode 56, the organic layer 57and the opposite electrode 58), and a capacitor 74. In this circuit, ateach intersection of the plural signal lines 71 and the scanning lines72, the organic TFTs 52, the organic EL display element 73 and thecapacitor 74 are disposed. The destinations to which the sourceelectrode, the gate electrode and the drain electrode in each of theselection TFT 52A and the drive TFT 52B are not limited to the caseillustrated in FIG. 7, and can be set freely.

In this organic EL display device, for example, when the organic ELdisplay element 73 is selected by the selection TFT 52A, this organic ELdisplay element 73 is driven by the drive TFT 52B. Thus, when anelectric field is applied between the pixel electrode 56 and theopposite electrode 58, light emission occurs in the organic layer 57. Inthis case, for example, in each of the adjacent three organic EL displayelements 73, light of red, green or blue is produced. Synthetic light ofthese lights is emitted via the opposite board 60 to the outside andthus, a tone image is displayed.

According to this organic EL display device, the organic TFT 52 has theconfiguration similar to that of the organic TFT described earlier andthus, the display performance can be enhanced, like the liquid crystaldisplay device.

Incidentally, the organic EL display device is not limited to the topemission type, and may be a bottom emission type that emits light viathe drive board 50, or may be a dual emission type that emits light viaboth of the drive board 50 and the opposite board 60. In this case, ofthe pixel electrode 56 and the opposite electrode 58, the electrode onthe side where the light is emitted is formed of a transmissivematerial, and the electrode on the side where the light is not emittedis formed of a reflective material.

2-3. Electronic Paper Display Device

The organic TFT is applied to, for example, an electronic paper displaydevice. FIG. 8 illustrates a cross-sectional configuration of theelectronic paper display device.

Incidentally, the device configuration (FIG. 8) and a circuitconfiguration to be described below with reference to FIG. 5 are merelypresented as examples, and can be modified as appropriate.

The electronic paper display device described here is, for example, anelectronic paper display in an active matrix drive system that uses theorganic TFT as a switching element. This electronic paper display deviceis, for example, a device in which a drive board 80 and an oppositeboard 90 including plural electrophoretic elements 93 are bondedtogether through an adhesive layer 100.

The drive board 80 is, for example, a board in which an organic TFT 82,a protective layer 83, a planarizing dielectric layer 84 and a pixelelectrode 85 are formed in this order on one surface of a supportingboard 81, and the plural organic TFTs 82 and pixel electrodes 85 arearranged in a matrix. The supporting board 81 is formed of glass, aplastic material or the like, and the organic TFT 82 has a configurationsimilar to that of the organic TFT described earlier. The protectivelayer 83 and the planarizing dielectric layer 84 are formed of, forexample, an insulating resin material such as polyimide, and the pixelelectrode 85 is formed of, for example, a metallic material such assilver (Ag). Incidentally, the pixel electrode 85 is connected to theorganic TFT 82 through a contact hole (not illustrated) provided in theprotective layer 83 and the planarizing dielectric layer 84. Whensufficient flatness is achieved by the protective layer 83, theplanarizing dielectric layer 84 may be omitted.

In the opposite board 90, for example, an opposite electrode 92 and alayer including the plural electrophoretic elements 93 are formed inthis order on one surface of a supporting board 91, and the oppositeelectrode 92 is formed all over the surface. The supporting board 91 isformed of, for example, a transmissive material such as glass or aplastic material, and the opposite electrode 92 is formed of, forexample, a transmissive conductive material such as ITO. Theelectrophoretic element 93 is, for example, an element in which chargedparticles are dispersed in an insulating liquid, and enclosed in a microcapsule. The charged particles include, for example, black particlesthat are graphite fine particles and white particles that are titaniumoxide fine particles.

In addition, the electronic paper display device may include, forexample, other components (not illustrated) such as a color filter.

The circuit for driving the electronic paper display device has, forexample, a configuration similar to the circuit of the liquid crystaldisplay device illustrated in FIG. 5. The circuit of the electronicpaper display device includes, in place of the organic TFT 22 and theliquid crystal display element 44, the organic TFT 82 and an electronicpaper display element (an element section including the pixel electrode85, the opposite electrode 92 and the electrophoretic element 93),respectively.

In this electronic paper display device, when the electronic paperdisplay element is selected by the organic TFT 82 and an electric fieldis applied between its pixel electrode 85 and opposite electrode 92, theblack particles or the white particles in the electrophoresis element 93are drawn to the opposite electrode 92 according to the electric field.As a result, contrast is expressed by the black particles and the whiteparticles and thus, a tone image is displayed.

According to this electronic paper display device, the organic TFT 82has the configuration similar to that of the organic TFT describedearlier and therefore, the display performance can be enhanced like theliquid crystal display device.

Up to this point, the present application has been described by usingthe embodiment, but the present application is not limited to thefeatures described for the embodiment, and can be variously modified.For example, the structure of the thin-film transistor of the presentapplication is not limited to the top-contact bottom-gate type, and maybe a top-contact top-gate type. In this case as well, a channel etchtype of organic TFT provided with an organic semiconductor layer (alower organic semiconductor layer and an upper organic semiconductorlayer) can be readily produced with stability and thus, effects similarto those of the top-contact bottom-gate type can be obtained.

Further, for example, the electronic device to which the thin-filmtransistor of the present application is applied is not limited to theliquid crystal display device, the organic EL display device or theelectronic paper display device, and may be other display devices. Forexample, as one of such other display devices, there is a MEMS (MicroElectro Mechanical Systems) display unit. In this case as well, thedisplay performance can be enhanced.

Furthermore, for example, the thin-film transistor of the presentapplication may be applied to electronic devices other than the displaydevice. For example, as one of such electronic devices, there is asensor matrix or a memory cell. In this case as well, the mobility andthe ON-OFF ratio of the thin-film transistor improve and thus, theperformance can be enhanced.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope and without diminishing itsintended advantages. It is therefore intended that such changes andmodifications be covered by the appended claims.

1. A thin-film transistor, comprising: an organic semiconductor layer;and a source electrode and a drain electrode spaced apart from eachother and disposed to respectively overlap the organic semiconductorlayer, wherein the organic semiconductor layer includes a lower organicsemiconductor layer, and an upper organic semiconductor layer formed onthe lower organic semiconductor layer and having solubility andconductivity higher than those of the lower organic semiconductor layer,and the lower organic semiconductor layer extends from a regionoverlapping the source electrode to a region overlapping the drainelectrode, while the upper organic semiconductor layer is disposed ineach of the region overlapping the source electrode and the regionoverlapping the drain electrode so that the respective upper organicsemiconductor layers are spaced apart from each other.
 2. The thin-filmtransistor according to claim 1, wherein an ionization potential of theupper organic semiconductor layer is smaller than an ionizationpotential of the lower organic semiconductor layer.
 3. The thin-filmtransistor according to claim 1, wherein the upper organic semiconductorlayer includes a material into which a substituent for giving solubilityto a material forming the lower organic semiconductor layer isintroduced.
 4. The thin-film transistor according to claim 1, whereinthe upper organic semiconductor layer includes a doping material forreducing resistance.
 5. A method of producing a thin-film transistor,the method comprising steps of: forming a lower organic semiconductorlayer; forming an upper organic semiconductor layer on the lower organicsemiconductor layer, the upper organic semiconductor layer havingsolubility and conductivity higher than those of the lower organicsemiconductor layer; forming a source electrode and a drain electrodespaced apart from each other and respectively overlapping the upperorganic semiconductor layer; and dissolving the upper organicsemiconductor layer selectively by using the source electrode and thedrain electrode as a mask.
 6. An electronic device comprising: athin-film transistor, including an organic semiconductor layer, and asource electrode and a drain electrode spaced apart from each other anddisposed to respectively overlap the organic semiconductor layer,wherein the organic semiconductor layer includes a lower organicsemiconductor layer, and an upper organic semiconductor layer formed onthe lower organic semiconductor layer and having solubility andconductivity higher than those of the lower organic semiconductor layer,and the lower organic semiconductor layer extends from a regionoverlapping the source electrode to a region overlapping the drainelectrode, while the upper organic semiconductor layer is disposed ineach of the region overlapping the source electrode and the regionoverlapping the drain electrode so that the respective upper organicsemiconductor layers are spaced apart from each other.